For the best format, go here: http://earlyvoting.net/
This blog disseminates information about early voting (voting before election day, at non-precinct locations), an increasingly common form of voting in the United States, and worldwide.
The blog is intended to be a resource to the election administration, reform, and research community.

... tells us very little about the relative rates of absentee ballot usage among whites, blacks, and hispanics.

It says what is says. For instance, among "predominantly Hispanic" jurisdictions, the average rate of absentee ballot request was 13.6 percent.

Those jurisdictions may vary in population size from densely urbanized counties in Arizona, New Mexico, and California to rural counties in Southwest Texas, northern New Mexico, southern Colorado, and northern Nevada.

Minimally, if we want to make a rough estimate of national averages, we need to weight the jurisdictions by the population in each county.